This might be useful to someone else in the Polymer community:
https://www.npmjs.org/package/karma-polymer-test
I needed to set up a test environment for my polymer elements so made a karma plugin. I've only been working with Node/Karma/mocha/chai for 2 days so will happily accept any things that need to be added to this thing.
All of our Polymer stuff is very much in pre-alpha so fair warning. I hope the unit tester might be of use to some people.
I need to add things like html loading and some other things so will be working on that as I maintain it.